1. Field of the Invention
The present invention relates to a method of controlling a blood pump. More specifically, the present invention relates to a method of controlling a blood pump used for medical equipment such as an artificial heart and an artificial heart-lung in which an impeller is supported by a magnetic bearing.
2. Description of the Background Art
A turbo pump such as a centrifugal pump or an axial flow pump, can be adapted to have higher performance and be smaller in size than a pulsatile type pump. Hence, the turbo pump has come to be used, for example, as a blood pump for an artificial heart. Flow rate in a turbo pump is generally controlled by changing the rotation speed of the pump.
FIG. 7 shows flow-pressure characteristic of a centrifugal pump. As shown by the solid line curve in FIG. 7, pressure in a typical prior art centrifugal pump hardly changes when the maximum flow rate only reaches a medium level.
FIG. 8 shows an exemplary use of the blood pump in which bypasses 2 and 3 are provided for a natural heart, and blood pump 4 assists the heart after surgical operation. Blood from lung 7 is supplied to blood pump 4 through bypass 3, and returned to aorta 15, pressurized by blood pump 4. The blood flows from aorta 15 through capillary vessel 5 and 6 to right atrium 13, from right atrium 13 to right ventricle 14, from right ventricle 14 to lung 7 and returns to left atrium 12.
In this case, flow rate of the pump depends on the discharge pressure of natural heart 1 and on fluctuation of pressure load caused by the variation in cannula resistance of the blood vessel. Reduction in pump flow rate causes thrombus in the pump, while excessive flow rate affects the organ. In order to suppress variation in the flow rate and achieve the desired characteristics as plotted by the dotted line in FIG. 7, it may be possible to make the tube narrower. However, in that case, pump efficiency degrades naturally. Such problem can be eliminated by realizing the characteristic represented by the straight line A in FIG. 7 having a constant inclination.